1. Field of the Invention
This invention relates to a thin film electroluminescent (EL) element and more particularly an improved high brightness low voltage drive thin film EL device, and a method of manufacturing the same.
2. Description of the Prior Art
A prior art dispersion type EL device using a powder of zinc sulfide (ZnS) type fluorescent substance could not be used as a light source of illumination because of its low brightness. In recent years, a thin film type EL device utilizing a thin film of fluorescent substance has become noted because of the high brightness.
The thin film EL device is now widely used for vehicles, display apparatus of a computer terminal element or the like and a source of illumination because the thin film EL device comprises a light emitting layer made of a transparent film which prevents dispersion of light incident from outside or emitted by an internal luminescent layer thereby decreasing halation and blur and providing clear display of high contrast.
For example, a thin film EL device utilizing Mn as an activator in ZnS has a dual dielectric structure wherein a transparent electrode made of a tin oxide layer (SnO.sub.2), a first insulating layer, a crystalline film utilizing ZnS as a host material and Mn acting as an activator, that is an MnS:Mn luminescent layer, a second insulating layer and a back electrode made of aluminum or the like are sequentially laminated on a transparent substrate.
Light is emitted as follows. When voltage is impressed across the transparent electrode and the back electrode, electrons that have been trapped at an interface level by an electric field induced in the luminescent layer are released and imparted with energy sufficient to accelerate the electrons so that these electrons collide against the orbit electrons of Mn (luminescent center) so as to excite the orbit electrons. Light is emitted when the excited activator returns to the base state.
As disclosed in Japanese Published Patent Specification Nos. 10358/1978 and 8080/1979, the electron beam vapor deposition method has been used for forming such luminescent layer as ZnS:Mn of the thin film EL device.
Such thin film EL device has been formed in a vacuum chamber 1 by irradiating a pellet 2 formed by sintering a mixture of ZnS and 0.1-1 at % of Mn with an electron beam 4 emitted by an electron gun 3 as shown in FIG. 10, so as to heat and vaporize the pellet and cause the vaporized pellet to deposit on a substrate 5.
With this method, however, since the vapor pressure of the host material comprising the luminescent layer, the vapor pressure of an element comprising the host material, and the vapor pressure of the activator (for example PZnS, PZn PS, PMn) differ greatly (PZnS&lt;PMn&lt;PZn&lt;PS), there are such problems that the host material of the laminated luminescent layer deviates from stoichiometric composition, thus degrading crystalline structure and making nonuniform the distribution of the activator due to nonuniform evaporation and reevaporation of elements once deposited on the substrate. In the foregoing description PZnS, PMn, PZn and PS respectively represent vapor pressures of ZnS, Mn, Zn and S.
Consequently, as shown in FIG. 11, the luminescent layer formed by the electron beam vapor deposition method has a particulate polycrystalline structure or at the initial stage of growth many small crystal particles are formed, that is a structure in which so called dead layers are present.
In a thin film EL device utilizing such luminescent layer, electrons E in the luminescent layer accelerated by an electric field applied from outside collide against the activator Im with the result that the electrons will be dispersed at the interfaces B of the crystal particles before the electrons contribute to luminescence. Thus, the electric field applied from outside does not efficiently contribute to luminescence.
Efficient deriving out of light emitted by the luminescent layer is important for increasing the luminescent efficiency. For efficiently deriving out the light from the luminescent layer, a method of controlling the refractive index and the film thickness of the first insulating layer has been proposed as, for example, in Japanese Published Patent Specification No. 55635 of 1983.
The equivalent circuit of such a thin film EL device can be represented by three serially connected capacitors (see FIG. 4) constituted by a first insulating layer 21, a luminescent layer 24 and a second insulating layer 25. When the specific dielectric constants .epsilon.r1 and .epsilon.r2 of the first and second dielectric layers are sufficiently larger than the specific dielectric constant .epsilon.l of the luminescent layer, that is .epsilon.rl, .epsilon.r2&gt;&gt;.epsilon.l, thin capacitances Cr1, Cr2, and Cl are expressed by a relation Cr1, Cr2&gt;&gt;Cl so that almost all portions of the voltage impressed across the element from outside will be applied across the luminescent layer with the result that it is impossible to obtain a high brightness with a low driving voltage.
For decreasing the driving voltage, it is advantageous to construct the first insulating layer with a material having a large dielectric constant. However, where a material having a large dielectric constant is used, the reflection at the interface between the first insulating layer and the transparent electrode becomes large, thus failing to efficiently derive out light from the luminescent layer.
For this reason, in order to obtain a practical brightness of about 20 ft-L, it necessary to apply a high voltage of the order of 200 V to the thin film EL device.